Fix signature validation
Fixes #4425: generate cookie validation keys in local config files.
Fixes #4566: Added client validation support for image validator
Removed index-test from codeception instructions since it's used automatically
Fixes #4622: Simplified the way of creating a Faker fixture template file
Add runner for all tests
Add namespaces to all tests
Use config variables instead constants. Cleanup redundant constants
Codeception test adjustments for basic and advanced applications

- Moved everything test-related into `tests` directory. Codeception tests are in `codeception`.
- Removed database module since we're using fixtures and migrations.
- Moved console entry points and bootstrap into `tests/codeception/bin`.
- Adjusted travis build scripts.
- Adjusted documentation to be consistent and reflect changes made.
Fixed templates and fixtures paths for advanced application
Removed db module from basic app, renamed _helpers to _support and _logs to _output
Renamed guy classes to actors, fixed phpdoc and code style
Exposed basic app entry URL via codeception.yml, refactored configs of advanced app to read from config and define entry script in a single place
Adjusted test configs structure
Fixed copyright year in batch files
Prefixed constants to avoid possible naming conflicts
Fixes #4757: fixed paths in advanced app init
Renamed YII_ROOT_DIR → YII_APP_BASE_PATH
Adjusted testing namespaces in application templates
Added description to each test config of advanced application